1. Field of the Invention
The present invention relates to an image forming method and apparatus in which a plurality of copy operations are performed on a single paper sheet.
2. Description of the Prior Art
Recently, copy machines have been provided with a plurality of developers storing toners of different colors so as to perform a plurality of copy operations on a single paper sheet, thereby forming a multiple super position, or multiple copy, mode. In this case, a paper sheet exhausted from a fixing device is returned to register rollers in front of a photosensitive drum so as to perform a multiple super position operation. In such a copy machine, a heat roller is used as the fixing device. The heat roller is formed into an hourglass shape in which diameters of two end portions thereof are larger than that of a central portion so as to apply tension on the paper sheet along a direction perpendicular to a feeding direction. Because of this, the paper sheet passing through the fixing device is wrinkled along the direction perpendicular to the feeding direction. When the paper sheet is returned so as to form a multiple super position, a paper jam easily occurs and a copy position can be undesirably shifted.
In addition to a multiple super position machine described above, a copy machine which copies on two sides of a single paper sheet by reconveying the sheet is known. However, in such a copy machine, the same problem as described above occurs.